Switch locks are known in the art including key locks (with a rotatable plug and tumblers) which are used to control use of an electrical switch. Such switch locks can be used, for example, to limit access to a computer or other electronic device by requiring that only a person with a key be able to turn on the power to the device. Wolniak et al. U.S. Pat. Nos. 4,405,843, 4,427,852, 4,566,167, 4,633,689 and 4,689,977, the disclosures of which are hereby incorporated by reference, illustrate various types of such switch locks.
In many instances, it is desirable to provide some readily noticeable indication of the position of the key lock. By making an operator more likely to notice when the lock is in, for example, a "power on" position, inadvertent breaches of security can be minimized.
A few locks with indicators have been proposed in the past. For example, devices have included a single light to indicate either one lock position (Moore et al. U.S. Pat. No. 1,145,206) or presence of the key in the lock (Thomas U.S. Pat. No. 1,544,048, in which removal of the key is prevented unless the lock is in a particular position).
Chaskin U.S. Pat. No. 2,286,463 shows a door lock provided with two lamps, one of which will be illuminated (only when the key is inserted) to indicate whether the door is locked or unlocked. A clumsy mechanical structure switches between the two lamps.